# Speed of light aboard a spaceship seen from Earth

1. May 3, 2015

### TheLil'Turkey

Let's say a spaceship made of a transparent material moving past the Earth in a straight line at a constant speed of 0.99 c, and it's at its minimum distance from Earth. If a light is turned on in the middle of the ship, for an observer on Earth will the light move towards the front of the ship at the speed c (with a speed of 0.01 c relative to the front of the ship) and towards the back of the ship at the speed c (with a speed of 1.99 c relative to the back of the ship?

If time moves more slowly on the ship for an observer on Earth, shouldn't light move equally slowly in any direction relative to the ship?

I'd like to understand the basics of special relativity; what should I read? The part of Feynman's lectures on physics that deals with this is all that I've read so far.

2. May 3, 2015

### Staff: Mentor

Light moves at $c$ relative to the ship. Velocities don't add linearly in relativity; see here:

If you apply this formula when $v = c$, you will see that it gives $s = c$ regardless of the value of $u$; so light moves at $c$ relative to all observers.